This invention relates generally to electric generating power systems and, more particularly, to over-voltage supression circuits for electric alternators.
Several techniques for providing over-voltage supression for electric generating machines are known. Such systems generally employ a plurality of silicon controlled rectifiers connected to the output windings of the machine for shunting the output windings in the event that an over-voltage condition occurs. Such representative prior art systems are described in U.S. Pat. Nos. 3,488,560 issued Jan. 6, 1970 to Richard L. Konopa and 3,581,150 issued Apr. 8, 1970 to Thomas F. Kirk et al.
An over voltage condition on an electric machine may occur, for example, as a result of a rapid change in the electrical load connected to the machine. Such a change may occur, for example, in an automobile alternator system when the battery has been accidently or otherwise disconnected, and subsequently a heavy current consuming device such as a blower motor is switched off. When the over-voltage condition is sensed by the normal voltage regulating system, the field excitation to the alternator is terminated. However, due to the inductance of the field winding and residual magnetism present in the alternator, a high voltage transient may be generated even though field excitation has ceased.
Accordingly, voltage protection systems have been developed for isolating the output windings of the alternator from the load connected thereto to prevent the over-voltage condition from damaging the accessories connected to the alternator, such as, in the case of an automobile, light bulbs and the radio and the like. The isolation is generally provided by connecting an electronic shunt switch across the output windings of the alternator to shunt the output voltage to a source of common potential upon the occurence of the over-voltage condition. Series switching systems have also been used to disconnect the alternator from the load when the over-voltage condition occurs.
Because of the high speed nature of the over voltage transients, the response time of the over-voltage protection circuitry must be made sufficiently fast to insure that the output windings are isolated from the load before any damage to the load can occur. In addition, particularly in voltage regulated systems, provision must be made to assure that the excitation of the field is terminated when the over-voltage protection circuitry is activated to prevent undue dissipation of power by the alternator and by the protection circuitry.
In prior art systems, particularly in those employing controlled rectifiers in the transient protection circuitry, a Zener or avalanche diode is generally employed to sense the output-voltage of the machine. The avalanche device is generally connected to the control or gate electrode of the controlled rectifier, and the output voltage of the machine operates to avalanche or break down the diode in the reverse direction when a predetermined level has been reached. The avalanche current flows through the diode and is applied to the gate electrode of the controlled rectifier to render the controlled rectifier conductive. The voltage regulator for the machine is provided with means for sensing the output voltage of the machine and for terminating the excitation of the field when the over-voltage condition occurs to prevent excessive power dissipation by the controlled rectifiers.
While these techniques provide over-voltage protection for an electric generating machine, problems may arise as a result of an undesired triggering of the protection circuitry occurring when no over-voltage condition is present. Such an undesired triggering may occur as the result of a voltage transient, heat or by a failure of one of the components in the protection circuit. In addition, due to the high surge currents being switched, a controlled rectifier may exhibit a delayed turn off characteristic and remain conductive for several cycles following the termination of a proper gate triggering signal. In prior art circuits, when an undesired triggering in the absence of an over-voltage condition or a delayed turn off occurs, the field excitation is not terminated by the voltage regulator because the triggering of a controlled rectifier causes the output voltage to drop below the regulating voltage. Under such circumstances, full field excitation is provided to the alternator, thereby causing the full output current of the alternator to be applied to the protection system, a condition which could result in damage to the protection system or to the alternator.
Other problems associated with prior circuits occur as a result of the substantial voltage drop across the avalanche device which triggers the controlled rectifiers. Because all of the triggering current for the controlled rectifiers must flow through the avalanche device, a relatively high power Zener diode must be used. In addition, the voltage appearing across the Zener diode results in a lower voltage drive to the controlled rectifiers, thereby making the triggering of the controlled rectifiers marginal in some cases.
Accordingly, it is an object of the present invention to provide an improved over-voltage supression circuit having an improved triggering system.
It is another object of the present invention to provide an improved over-voltage protection system that deenergizes the field of the machine whenever the over-voltage protection system is activated, regardless of the cause of the activation.
Another object of the present invention is to provide an over-voltage protection system for an electric machine that is more reliable than systems according to the prior art.
In accordance with the preferred embodiment of the invention, a plurality of controlled rectifiers are connected to the output windings of the machine to shunt the output windings to a source of common potential when the controlled rectifiers are rendered conductive. Impedance means, such as a low value resistor or semiconductor diodes are connected in series with the controlled rectifiers to provide a means for sensing the current flowing through the controlled rectifiers. The voltage regulator is connected to the impedance means and is responsive to current flowing therethrough to prevent excitation of the field when the controlled rectifiers are in a conductive state.
The triggering circuit for the controlled rectifiers comprises a sensing circuit including a Zener diode or other breakdown device and a transistor switch connected to the sensing circuit and to the control electrodes of the controlled rectifiers.
When the output voltage of the machine reaches a value sufficient to break down the Zener diode, the transistor switch is rendered conductive to provide a gate control signal to the controlled rectifiers, thereby rendering the gate controlled rectifiers conductive. The current flowing through the controlled rectifiers causes a voltage to appear across the impedance means, and this voltage is sensed by the voltage regulator to prevent further excitation of the field. Sensing the conduction of each controlled rectifier at the output terminal thereof provides a positive indication of current flow through the controlled rectifier, thereby causing the excitation of the field to be terminated regardless of the event that caused the rectifier to become conductive. The transistor switch provides a positive gate control signal to the controlled rectifiers to assure positive turn-on.